The present invention relates to a solid electrolytic capacitor using a conductive polymer compound as a solid electrolyte and a method of manufacturing the same and, more particularly, to a solid electrolytic capacitor having a high capacitance reproduction ratio and excellent in frequency characteristics and heat resistance, and a method of manufacturing the same.
Compactness and high reliability of electronic devices have been demanded along with developments of scientific techniques. Demand has arisen for a highly reliable large-capacitance solid electrolytic capacitor excellent in characteristics up to a high-frequency range. To meet this demand, extensive research and development have been made.
A general solid electrolytic capacitor has a structure in which a molded porous body of a valve metal such as tantalum or aluminum serves as a first electrode (anode), an oxide coating of this metal serves as a dielectric, and a solid electrolyte such as manganese dioxide (MnO.sub.2) or a 7,7',8,8'-tetracyanodimethane (TCNQ) complex salt serves as part of a second electrode (cathode). In this case, the solid electrolyte must have a function of electrically connecting electrode leads to the entire surface of the dielectric inside the molded porous body and a function of repairing short-circuiting caused by a dielectric coating defect. As a result, a metal having a high electrical conductivity but having no dielectric repair function cannot be used as a solid electrolyte. Manganese dioxide or the like subjected to transition into an insulator due to heat caused by a short-circuiting current has been used. However, when manganese dioxide is used as part of an electrode, the impedance cannot be decreased in a high-frequency range because manganese dioxide does not have a sufficiently high electrical conductivity. When the TCNQ complex salt is used as part of an electrode, the TCNQ complex salt tends to be thermally decomposed, resulting in a low heat resistance.
In recent years, new materials have been developed in the field of polymers. A conductive polymer obtained by adding (doping) an electron donor or acceptor compound (dopant) to a conjugated polymer such as polyacetylene, polyparaphenylene, polypyrrole, or polyaniline. A 5-membered heterocyclic compound (e.g., polypyrrole or polythiophene) or polyaniline can be electrolytically polymerized to easily obtain a conductive polymer and is used as the electrolyte of a capacitor (Japanese Patent Laid-Open No. 64-36012 as Reference 1 and Japanese Patent Laid-Open No. 3-64013 as Reference 2). According to this method, however, electrolytic polymerization is performed on an insulating oxide coating to make it very difficult to form a uniform conductive polymer film. For this reason, a method of forming a conductive polymer electrolytically polymerized on an oxide coating with a conductive precoat film is popularly used (Japanese Patent Laid-Open Nos. 64-32619, 64-74712, 1-225110, and 2-117121, and References 1 and 2). In this method, however, an auxiliary electrode for electrolytic polymerization must be located near a capacitor element, resulting in poor mass production.
There is also proposed a method of applying and drying a conductive polymer soluble in an organic solvent to obtain a solid electrolyte. For example, a polyaniline solution obtained in advance by a method of polymerizing aniline is applied to and dried on the surface of a metal oxide coating, and the dried polyaniline coating serves as a solid electrolyte, thereby proposing a solid electrolytic capacitor (Japanese Patent Laid-Open No. 3-35516). According to this method, however, the polyaniline solution has a very high viscosity and cannot be permeated into the entire fine oxide coating having a large area. For this reason, only a capacitor having a very low capacitance reproduction ratio (i.e., a ratio of an actual electrostatic capacitance value to a design value) can be produced. To the contrary, there is also proposed a method of polymerizing an aniline monomer on an oxide coating to form polyaniline. In this case, although a sufficiently high capacitance reproduction ratio can be obtained, the capacitor characteristics in the high-frequency range are poorer than that of a capacitor using polypyrrole because polyaniline itself has a lower electrical conductivity than that of polypyrrole.